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TMessagesProj/jni/voip/webrtc/rtc_base/timestamp_aligner.h

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+/*
+ *  Copyright (c) 2016 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_TIMESTAMP_ALIGNER_H_
+#define RTC_BASE_TIMESTAMP_ALIGNER_H_
+
+#include <stdint.h>
+
+#include "rtc_base/system/rtc_export.h"
+#include "rtc_base/time_utils.h"
+
+namespace rtc {
+
+// The TimestampAligner class helps translating timestamps of a capture system
+// into the same timescale as is used by rtc::TimeMicros(). Some capture systems
+// provide timestamps, which comes from the capturing hardware (camera or sound
+// card) or stamped close to the capturing hardware. Such timestamps are more
+// accurate (less jittery) than reading the system clock, but may have a
+// different epoch and unknown clock drift. Frame timestamps in webrtc should
+// use rtc::TimeMicros (system monotonic time), and this class provides a filter
+// which lets us use the rtc::TimeMicros timescale, and at the same time take
+// advantage of higher accuracy of the capturer's clock.
+
+// This class is not thread safe, so all calls to it must be synchronized
+// externally.
+class RTC_EXPORT TimestampAligner {
+ public:
+  TimestampAligner();
+  ~TimestampAligner();
+
+  TimestampAligner(const TimestampAligner&) = delete;
+  TimestampAligner& operator=(const TimestampAligner&) = delete;
+
+ public:
+  // Minimum difference of two timestamps generated by
+  // "TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us)"
+  // This avoids the caller from getting two timestamps with the same
+  // millisecond.
+  static constexpr int64_t kMinFrameIntervalUs = rtc::kNumMicrosecsPerMillisec;
+
+  // Translates timestamps of a capture system to the same timescale as is used
+  // by rtc::TimeMicros(). `capturer_time_us` is assumed to be accurate, but
+  // with an unknown epoch and clock drift. `system_time_us` is
+  // time according to rtc::TimeMicros(), preferably read as soon as
+  // possible when the frame is captured. It may have poor accuracy
+  // due to poor resolution or scheduling delays. Returns the
+  // translated timestamp.
+  int64_t TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us);
+
+  // Returns the translated timestamp without updating the states. This is to
+  // allow TimestampAligner to translate capturer time into system clock based
+  // on earlier observations. It won't guarantee monotonicity.
+  int64_t TranslateTimestamp(int64_t capturer_time_us) const;
+
+ protected:
+  // Update the estimated offset between capturer's time and system monotonic
+  // time.
+  int64_t UpdateOffset(int64_t capturer_time_us, int64_t system_time_us);
+
+  // Clip timestamp, return value is always
+  //    <= `system_time_us`, and
+  //    >= min(`prev_translated_time_us_` + `kMinFrameIntervalUs`,
+  //           `system_time_us`).
+  int64_t ClipTimestamp(int64_t filtered_time_us, int64_t system_time_us);
+
+ private:
+  // State for the timestamp translation.
+  int frames_seen_;
+  // Estimated offset between capturer's time and system monotonic time.
+  int64_t offset_us_;
+
+  // State for the ClipTimestamp method, applied after the filter.
+  // A large negative clock drift of the capturer tends to push translated
+  // timestamps into the future. `clip_bias_us_` is subtracted from the
+  // translated timestamps, to get them back from the future.
+  int64_t clip_bias_us_;
+  // Used to ensure that translated timestamps are monotonous.
+  int64_t prev_translated_time_us_;
+  // Offset between `prev_translated_time_us_` and the corresponding capturer
+  // time.
+  int64_t prev_time_offset_us_;
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_TIMESTAMP_ALIGNER_H_